Technical Pressure Question

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A few reasons, already covered
1 being under pressure forces air out of the system when filling
2 being under pressure keeps air out of the system
3 being under pressure raises the boiling point of water (water boils at 100°c at 1 bar)
4 most boilers use a pressure switch to prove water in the system
1-1.5 bar is used as it is ideal for most family homes as most are 2 to 3 story and keeps prices down
It is the buildings height that governs the amount of pressure you need
we look after some systems pressurised to 5 or 6 bar

Matt
 
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R

rebuilder

A few reasons, already covered
1 being under pressure forces air out of the system when filling
2 being under pressure keeps air out of the system
3 being under pressure raises the boiling point of water (water boils at 100°c at 1 bar)
4 most boilers use a pressure switch to prove water in the system
1-1.5 bar is used as it is ideal for most family homes as most are 2 to 3 story and keeps prices down
It is the buildings height that governs the amount of pressure you need
we look after some systems pressurised to 5 or 6 bar

Matt

Great points. Number 4. How do you calculate the pressure needed for the height of the building? If a building is 40 metres high (4 bar is pressure from top to bottom), how do you calculate the system pressure? If it is set to 1 bar a ground level what will be the pressure at the top 40 metres above?
 

MrV

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Yes but we are talking about pressureabove atmospheric.
surely it's only relative on an open vent system. If its a sealed system, surely the only thing that affects existing pressure is temperature.
 
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If it is set to 1 bar a ground level what will be the pressure at the top 40 metres above

if it is set to 1 bar at ground level then you wont have any water 40 metres above never mind pressure

as a general rule of thumb 1 bar will lift a column of water 10 metres

so above you would need at least 4 bar
 
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In a sealed boiler the boiling point of water is about 120 C @ 1.0 Bar.

Thats increased to 127 C @ 1.5 Bar.

So thats why I recommend setting most boilers to 1.5 Bar cold.

I am surprised that there seem to be so many misconceptions being voiced above.

Tony
 
R

rebuilder

If it is set to 1 bar a ground level what will be the pressure at the top 40 metres above

if it is set to 1 bar at ground level then you wont have any water 40 metres above never mind pressure

as a general rule of thumb 1 bar will lift a column of water 10 metres

so above you would need at least 4 bar

Obvious ;) Thanks.

The first post was asking why a system in a two floor house is set to 1 bar, when 0.5 bar, or less, would be fine for the setting when cold. If a system is boiling then something is wrong and needs attention, hence I do not see why it should be over pressurized in case it boils. Have I missed something?

Is is the pressure vessel pressure set to equal the system pressure when cold?

Boy I must be bored. But this is interesting.
 
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Is is the pressure vessel pressure set to equal the system pressure when cold?

Boy I must be bored. But this is interesting.

No, slightly below (cold system pressure x 0.8 )

I just remembered I had this picture that some may find it of interest
View media item 41637this is the energy centre of a 43 metre high building and this is the heating expansion vessel for the entire building (800L)
the pressuisation unit to the left plays no part as such in pressurising the system its only role is to top up the system

you set the req pressure and building height etc on the dial and the top mounted compressor will put air in as req or the solenoid operated air valve will let air out as req,
if you look closely at the front foot of the EV you will see a wire running to the pressurisation unit this is connected to a weight operated switch behind the foot that closes when the EV is below a quarter empty, this then operates the pressurisation units pumps which then add more make up water as req

there was originally a problem with the system when it was installed as the installer had installed the blow off pipework with fixed pipework (out of view in pic) which prevented the EV from "rocking" and therefore the foot switch operating

clever system though and it means you can use a much smaller EV

Matt
 
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Yes but we are talking about pressureabove atmospheric.

Interesting. So are you saying a gauge reading 1 bar at ground level is really 2 bar? The pressure in the system plus atmospheric which is 1 bar?

It's 1 barg (gauge) if it's referenced to atmospheric pressure and 1bara (absolute) if it's referenced to a complete vacuum. 1 barg = 2 bara if the ambient pressure is 1bar. All pressures will always be quoted as barg.
 
R

rebuilder

Matt,

Very interesting. A few points:

Is the pressurization unit an air and water unit doing both?

Why does the EV need to "rock"?

It would have thought the size of an expansion vessel is fixed, so how does it get away with having a smaller vessel?

Do you set the height of the building and volume of the system and the pressurization unit does the rest?
 
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There are a few more points included in the uk.d-i-y faqs that are relevant and haven't been mentioned.

The advantages of sealed systems are at least the following:

  • They are simpler to install since there is no header tank in the roof space and any of its possible problems such as over-flowing or freezing.
    They are easier to fill since air is expelled under pressure when bleeding the radiators.
    There can be no problems that stem from a number of installation faults occurring on open systems that end up drawing air into the system causing air locks, corrosion and sludge.
    The increased pressure raises the boiling point of water in the boiler. In an older boiler hot-spots cause localised boiling that often make kettle-like noises or even loud clanks or bangs.
    In the unlikely event of major damage to the heating system the resulting flood is limited. In contrast with a conventional system where the automatic top-up on the header tank will contribute an indefinite amount.
    They save space in flats where there is usually no good location for the header tank anyway.
    The system can be flushed out under pressure using the mains water supply.

In fairness the following are slight draw backs:

  • It is a bit harder to introduce chemicals (such as inhibitors and cleansers).
    The system pressure needs checking from time to time."
 
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Why does the EV need to "rock"?

There's an electronic load cell under the front leg detecting the "weight" of the vessel.
Attatched fixed pipework would interfere with this "load sensing".
Well maybe its not that advanced. :mrgreen:

The "rocking" movement will be small but attatched pipe work would likely interfere with this necessary movement as in this case.
 
G

gaswizzard

What you have to remember here is that for every 0.5 bar increase in fill pressure the EV capacity would have to be doubled in size. :p
 
R

rebuilder

What you have to remember here is that for every 0.5 bar increase in fill pressure the EV capacity would have to be doubled in size. :p

Is "fill pressure" the pressure of the vessel set when there is no pressure in the system, or the system pressure?

Thanks
 
R

rebuilder

Interesting stuff. This states "nitrogen". Is air filling the vessel a different pressures. One poster says the charge must be 0.8 of the static head, now this states 0.5 above if not spot on.

So in a flat. One poster says fill to 0.8 bar and this states 0.5 bar (but with nitrogen). I am confused a little. I think this thread is getting there :)

Back to the original post. In a house with a boiler in the kitchen, the highest point will be the top of the cylinder above - about 2.5 metres which is 0.25 bar. So the vessel in a normal house is set to 0.5 bar. one poster says about 0.2 bar.
 

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